1. Field of Invention
Embodiments of the invention relate generally to uninterruptible power supplies (“UPS”). More specifically, at least one embodiment relates to an apparatus and method for employing a DC source with an uninterruptible power supply.
2. Discussion of Related Art
Uninterruptible power supplies are often used to supply power to critical loads to reduce the risk that those loads will experience an unplanned power outage. Generally, a primary power supply (e.g., power supplied from a utility) is supplied to an input of the UPS and critical loads are connected to an output of the UPS. Functionally, an on-line UPS typically operates by converting an alternating current (“AC”) normal supply to direct current (“DC”). The DC is used to continuously charge batteries and is also supplied to an inverter that converts the DC back to AC providing a regulated AC voltage to the output of the UPS. The batteries in the UPS provide power to the inverter for conversion to AC for a short period of time, for example, when the primary source of power is not available.
Two different UPS topologies are depicted in FIGS. 1 and 2. In FIG. 1, a traditional double-conversion type UPS 100 is shown. The UPS 100 includes an input module 102 (e.g., a rectifier), an input 103, an output module 104 (e.g., an inverter), an output 105, and batteries 106. It is referred to as a double conversion topology because AC input power is converted to DC power in the input module 102 (a first energy conversion) and DC power is converted to AC in the output module 104 (a second energy conversion). In addition to the UPS 100, a UPS system 107 may also include a generator 108 and a transfer switch 110 (e.g., an automatic transfer switch).
In operation, power is supplied from the power source 112 to a non-critical load 114 via the transfer switch 110 where the circuit supplying the non-critical load 114 may be included as part of the UPS. During most periods, power is supplied to critical load or loads 116 via the transfer switch 110 and UPS 100. The power supplied to the input 103 is converted to DC by the input module 102 and supplied to a DC bus 118. Power is supplied from the DC bus 118 to the batteries 106 to maintain a float voltage on the batteries that maintains their charge. Power is also supplied from the DC bus 118 to the output module 114 to supply power to the critical load 116 (e.g., a load that is intended to receive continuous regulated power).
When a loss of the normal power source 112 is detected (by, for example, sensing logic in the UPS 100) a start signal is provided to start the standby generator 108. Power from the batteries 106 is supplied to the output module 104 to supply power to the critical load 116 connected to the output 105 of the UPS during the period when primary power is unavailable and the generator 108 is not yet producing rated output voltage. When the generator 108 is producing rated voltage, the transfer switch 110 operates to disconnect the normal source 112 from the UPS 100 and connect the generator to the input 103 of the UPS 108. The generator 108 supplies the power for the critical loads 116 and maintains a charge on the batteries 106. Because the UPS includes batteries, there is no interruption in the power supplied to the UPS output 105 when the primary power is lost and during the transition to the generator 108. Conversely, an interruption in the power supplied to the non-critical loads 114 does occur when the primary power is lost because they are not connected to the output 105 of the UPS. The non-critical load 114 remains without power until the transfer switch 110 operates to connect the generator 108 to the non-critical load 114. If necessary, the critical load 116 will continue to operate while being supplied with power originating from the generator so long as the generator is operating, e.g., has fuel. When the primary power source 112 is again available, the transfer switch 110 operates to disconnect the generator 108 and reconnect the primary power source 112 to the UPS input 103.
Referring now to FIG. 2, another UPS topology is depicted. The UPS 200 of FIG. 2 also includes an input module 202, an input 203, an output module 204, an output 205, and batteries 206. The UPS in FIG. 2, however, includes a boost converter 219 as part of the input module 202. The topology shown in FIG. 2, is provided, for example, in the SYMMETRA® UPS line manufactured by American Power Conversion Corporation of West Kingston, R.I.
The input module 202 can include a power factor correction circuit that may be included in the boost converter 219. The power factor correction circuit allows the UPS 200 to draw power from the primary power source 212 while maintaining a favorable relationship between the current and the voltage at the input 203, i.e., maintain approximately a unity power factor. The boost converter 219 provides a method by which the batteries 206 can be connected to the UPS 200 in conjunction with the power factor correction circuit.
The UPS 200 may be included in a UPS system 207 that also includes a generator 208 and a transfer switch 210. In operation, power is supplied from a power source 212 to the non-critical load 214 via the transfer switch 210 where the circuit supplying the non-critical load 214 may be included as part of the UPS. Here too, during most periods, power is supplied to critical load 216 via the transfer switch 210 and UPS 200. The power supplied to the input 203 is converted to DC by the input module 202 and supplied to a DC bus 218. Power is supplied from the DC bus 218 to the output module 204 to supply power to the critical load 216 connected to the UPS output 205.
The UPS system 207 in FIG. 2 responds to a loss of the primary source of power in a manner that is similar to the response of the UPS system 107. More specifically, when a loss of the normal power source 212 is detected, a start signal is provided to start the standby generator 208. Power from the batteries 206 is supplied to the output module 204 to supply power to the critical load 216 during the period when primary power is unavailable and the generator 208 is not yet producing rated output voltage. After the generator 208 is producing rated voltage, the transfer switch 210 operates to disconnect the normal source 212 from the UPS 200 and connect the generator to the input 203 of the UPS 208. Because the UPS includes batteries, there is no interruption in the power supplied to the UPS output 205 when the primary power is lost and during the transition to the generator 208. Conversely, an interruption in the power supplied to the non-critical loads 214 does occur when the primary power is lost because they are not connected to the output 205 of the UPS. Non-critical load 214 remains without power until the transfer switch 210 operates to connect the generator 208 to the non-critical load 214.
When the primary power source 212 is again available, the transfer switch 210 operates to disconnect the generator 208 and reconnect the primary power source 212 to the UPS input 203 and the non-critical loads 214. Once again, the critical loads 216 do not experience a loss of power when the transfer switch 210 operates, but the non-critical loads 214 temporarily lose power when the transfer switch 210 disconnects the generator 208 and transfers the non-critical loads 214 back to the primary source of power 212.
Typically, the electrical ratings (e.g., voltage, current, power, etc.) of the UPS (e.g., 200) correspond to the requirements of the power system in which it is installed. When, for example, the critical load 216 connected to the UPS 200 is not expected to exceed a maximum of 500 kilowatts the input module 202 and the output module 204 of the UPS 200 can be rated for a minimum of 500 kilowatts, although to provide a margin, a UPS generally has a capacity that exceeds the expected maximum demand by some percentage. Electrical ratings can be increased by, for example, increasing the current or voltage rating of the UPS components, and/or by adding input modules 202 and output modules 204. In general, increases in the UPS ratings will increase the cost of the UPS 200 because of the increased cost of higher rated power electronics required for the UPS. As a result, users tend to minimize the amount of load that is supplied by the UPS 200 to the extent possible, and select a UPS 200 that meets the expected load requirements of the critical loads 216 and no more, to minimize the power requirements (and correspondingly, the cost) of the UPS 200.
One result of the existing approaches is that AC generators are typically used with a UPS when a generator is employed with a UPS because the input to the UPS is AC. Employing an AC generator instead of a DC generator increases the cost and complexity of these approaches, however, because a transfer switch is required to switch between the AC sources that supply power to the UPS. In addition, AC generators are synchronous machines that run at a fixed speed related to the system frequency. As a result, AC generators typically run slower than the speeds at which DC generators routinely operate. An increase in generator speed results in an increase in the generator power rating and a corresponding decrease in the cost per kilowatt of electricity generated; therefore, it is advantageous to employ DC generators.